The present invention relates to nonwoven web and film laminates with improved strength. More particularly, the present invention relates to laminates for use in disposable garments and personal care products with improved tear resistance, and to a method of manufacturing such laminates.
Industry has long recognized the benefits of combining barrier properties of films and cloth-like attributes of nonwoven fabrics for various medical, personal care and commercial applications. Furthermore, such web/film laminates may also exhibit certain levels of elasticity, and when incorporating stretched filled microporous film, breathability. Therefore, laminates have been produced using both film and nonwoven web materials.
Lamination of films has been used to create materials which are both impervious and somewhat cloth-like in appearance and texture. Uses for such laminates include the outer covers for personal care products such as diapers, training pants, incontinence garments, and feminine hygiene products. In this regard, reference may be had to coassigned U.S. Pat. No. 4,818,600 dated Apr. 4, 1989 and U.S. Pat. No. 4,725,473 dated Feb. 16, 1988. Additionally, such materials are particularly suited for use in protective outer wear such as coveralls, and surgical garments and drapes. See in this regard coassigned U.S. Pat. No. 4,379,102 dated Apr. 5, 1983.
A primary purpose of the film in such laminations is to provide barrier properties. There is also a need for such laminates to be breathable so that they have the ability to transmit moisture vapor. Apparel made from laminates of these breathable or microporous films are more comfortable to wear by reducing the moisture vapor concentration and the consequent skin hydration underneath the apparel item.
Despite exhibiting many positive attributes, when used inappropriately or when exposed to particularly stressful conditions, laminates sometimes tear. In an attempt to create a nonwoven laminate with improved barrier properties, improved strength and with elastic attributes, but at lower costs, laminates have been developed in which the web fiber size has been reduced and polymer molecular weight distribution has been narrowed (since it affects polymer mechanical properties). For instance, it has been suggested that propylene polymers having high melt flow rate and narrow molecular weight distribution can be used to produce fibers for nonwoven webs and fabrics having superior barrier properties, tensile strength and softness. For example, U.S. Pat. No. 5,529,850 to Morini et. al. describes the preparation of crystalline polypropylene polymers having narrow molecular weight distribution, through the use of specific di- or polyesters as internal or external electron donors in polymerization reactions accompanying a catalyst component, such as an active magnesium halide and a titanium compound and al-alkyl compounds.
U.S. Pat. Nos. 5,726,103 and 5,763,080 to Stahl et al. describe fibers and fabrics incorporating lower melting propylene polymers in order to achieve a relatively strong and relatively fluid impervious fabric. In particular the Stahl patents describe propylene homopolymers and copolymers formed by metallocene catalyst systems. Such propylene polymers exhibit generally lower melting behavior than non-metallocene catalyzed propylene polymers. Stahl indicates that this low melting behavior is of use in the fabrication of fibers and fabric that depend on lower melting behavior or upon melting point differential between two fabrics to achieve bonding. Such fibers would include chenille or tufted, core and sheath. Stahl indicates that fabrics such as spunbond and meltblown nonwovens, when combined in spunbond/meltblown/spunbond (SMS) fabrics will show bonding at lower temperatures, and in particular, allow for the making of a higher melting fiber into a meltblown and a lower melting fiber into a spunbond. In the prospective examples of the Stahl patents, Stahl indicates that the overall strength of the fabric samples utilizing metallocene-catalyzed polypropylene in the spunbond layers will be as high as controls (which are unbonded SM fabrics). In a further prospective example utilizing one metallocene-catalyzed homopolymer polypropylene xe2x80x9cSxe2x80x9d layer and a commercial 1100 mfr polypropylene xe2x80x9cMxe2x80x9d layer, the prospective fabric would have improved barrier and filtration properties with no loss of laminated fabric strength when compared to the control. Each of these patents do not provide for better than expected tear strength in a film/nonwoven laminate.
U.S. Pat. No. 5,723,217 to Stahl et al. describes polyolefin fibers and their fabrics. This Stahl patent discusses fibers made from reactor grade isotactic poly-alpha-olefin wherein polypropylene is produced by single site catalysis. Stahl asserts that the polypropylene fibers produced will generally be stronger or have higher tenacity than conventional polymer when drawn to a fine diameter. Stahl also asserts that meltblown and or spunbond fabric containing the fiber will gain extra strength but does not allude to any method for creating a breathable film laminate with enhanced tear strength.
U.S. Pat. No. 5,612,123 to Gessner et al. describes a distribution enhanced polyolefin product. In particular this patent discusses that improved meltspinning productivity is achieved by employing polyolefin resins having key molecular weight distributions and Theological property parameters within predetermined ranges. Such polyolefin filaments and the single layer spunbond fabric prepared by the process exhibited high tenacity and tear property values. This patent also fails to allude to a method for increasing the tear properties of a breathable film laminate.
U.S. Pat. No. 5,464,688 to Timmons et al. describes nonwoven web laminates with improved barrier properties. Such webs are formed with commercially acceptable polymer with reduced molecular weight distribution in the meltblown layer of an SMS.
While metallocene-catalyzed polypropylene has heretofore been used in laminates, specifically as part of stretch bonded laminates and necked bonded laminates, the structural components, physical attributes and bonding processes of these laminates are markedly different from breathable film laminates. Furthermore, tear measurement tests, such as grab tensile/peak energy for necked bonded (NBL) and stretch bonded laminates (SBL), as well as a single spunbond layer show a higher peak energy value (in the machine direction) for Ziegler-Natta catalyzed polypropylene spunbond than for metallocene-catalyzed spunbond in these laminates. One would therefore not expect that spunbond with narrow molecular weight distribution would significantly increase tear strength in a breathable film/nonwoven web laminate.
Therefore, despite the improvements in the nonwoven laminate area, there exists a need for a breathable film/nonwoven web laminate which demonstrates increased tear strength without the addition of significant cost. Further, there exists a need for a method for producing such a laminate composite which can be done in-line at high speeds and over a short time span. Finally, there is a need for personal care products and other garments which utilize such laminates in their composite constructions. It is to the provision of such composite and method that the present invention is directed.
An object of the present invention is to provide a nonwoven web/film laminate material which exhibits significant tear strength attributes.
A still further object is to provide a nonwoven web/film laminate embodying the above-discussed features which utilizes relatively inexpensive materials to increase strength properties.
A still further object is to provide an in-line process for preparing a nonwoven web/film laminate which allows for increased tear strength in the finished laminate.
A specific object resides in providing a material having many of the previously identified attributes which can be advantageously used in personal care products.
The present invention relates to a film/nonwoven web laminate including at least one nonwoven web layer having a narrow molecular weight distribution and a film.
In one embodiment of the present invention, the film is a stretched microporous film that includes an elastomeric resin and a film filler.
The present invention is also directed to a process for producing a laminate including at least one nonwoven web layer having a narrow molecular weight distribution and a film including the steps of forming a nonwoven web of a metallocene-catalyzed polypropylene and bonding a film layer to the newly formed nonwoven web layer within 1-30 seconds of the formation of the nonwoven web layer.